Wind turbines typically utilize a plurality of blades which are caused to be turned by the force of wind passing over the blades. Rotation of the blades by the wind drives a central shaft which is typically connected to an electrical generator.
To prevent an overspeed condition during storms or high wind conditions, some turbines have the capability to vary the pitch of the blades to reduce the rotational speed of the turbine. This prevents the turbine from rotating at an unsafe speed, but such systems are complex and expensive.
Other types of systems have also been used for preventing overspeed conditions. For example, U.S. Pat. No. 4,180,372 describes a brake system which involves a spring-loaded plate which is hinged on the tip of the rotor blade in a manner such that it rotates outwardly due to centrifugal force resulting from overspeed conditions. Such system is limited to being mounted on the very tip of the turbine blade.
U.S. Pat. No. 2,074,149 describes a braking system in a propeller in which the tips of the propeller can rotate due to inertial loads of a rotating blade on a cam surface.
U.S. Pat. No. 2,126,202 describes a governor for a wind motor. The device includes paddles on the outer ends of a barrel secured at its center to the hub of the motor. As the rotational speed increases, the paddles are turned or rotated so that they present resistance to movement through the air, thereby retarding rotational speed of the wind motor.
U.S. Pat. No. 2,302,054 describes a variable pitch wind motor in which the blades can be turned to prevent an overspeed condition even in high winds.
U.S. Pat. No. 2,437,659 describes a balanced impeller structure for wind motors. Spoilers are mounted externally to the blades and they can be rotated in a plane perpendicular to the plane of blade rotation.
U.S. Pat. No. 2,493,895 describes a propeller blade having flaps hingedly attached to its surface. The flaps are caused to pivot outwardly into the air when the propeller rotates at very high speed.
U.S. Pat. No. 4,082,479 describes an overspeed spoiler for vertical axis wind turbines. The spoiler is hinged to the airfoil and is held against the surface of the airfoil with a spring. At a predetermined rotor speed the spoiler pivots against the spring force and becomes transverse to the air flow to increase drag.
U.S. Pat. Nos. 4,297,076 and 4,374,631 describe a wind turbine in which the tip portions of the blades are variable in pitch to control yaw and relieve bending moments on the blades. The tips also control overspeed.
U.S. Pat. No. 4,355,955 describes a wind turbine rotor speed control. The blade tips can be rotated or pivoted in response to movement of tabs pivotally attached to the trailing edges of the blade.
U.S. Pat. No. 4,431,375 describes a wind-driven generator having variable pitch blades.
U.S. Pat. No. 4,500,257 describes a spoiler system in which the spoilers on the blades are initially stored near the rotor axis. A latch releases the spoilers and centrifugal force causes the spoilers to move up the turbine blades away from the rotor axis to produce a braking effect.
U.S. Pat. No. 4,571,156 describes an air foil with a trailing spoiler which is flexible and resilient. The portion of the spoiler that extends past the trailing edge bends toward the wind during high wind conditions to prevent overspeeding.
U.S. Pat. No. 4,867,642 describes a wind mill with an air brake. The wing tip is automatically turned out when the speed of rotation of the wing exceeds a certain level. There has not heretofore been described an aerodynamic braking device having the features and advantages provided by the present invention.